An electrically powered tool, such as a portable rotary hand held saw and the like, typically includes an electric motor for providing a source of power which is transmitted through a gear train to the saw blade or other implement for performing the intended work. As can be appreciated, there is a large amount of force exerted on the implement as it is worked through the material. This force, which is generated by the electric motor, is conducted through the gear train to the implement. Therefore, the gear train and the electric motor are necessarily designed to withstand the high stresses resulting from these forces.
The loads, and therefore the stresses, experienced by the gear train and the motor vary with the cutting conditions. When the power tool is freewheeling, i.e., the tool is operating but the implement is not cutting through material, the loading of the gear train is fairly low. When the tool is cutting easily through a piece of material, the loading of the gear train is still of fairly low magnitude. However, if the tool bogs down or "stalls" in the material, i.e., the implement is no longer moving smoothly through or is stuck in the workpiece, the loading and stresses acting on the gear train increase.
Although the implement may have "stalled" in the workpiece, the driving torque from the electric motor will continue and the force acting on the gear teeth may tend to force the gears apart and drive the armature shaft radially outwardly. If the armature shaft should deflect radially, the loading condition on the gear train will be further aggravated because the driving connection thereon will be via the tips of the gear teeth instead of against the faces of the gear teeth.
Therefore, to ensure that the gear teeth do not fracture or otherwise fail under such loading conditions, and hence, to ensure a long reliable life for the power tool, the gears are made from hardened steel. Hardened steel gears, however, are typically more difficult to manufacture and are more expensive as compared to other types of gears such as powdered metal gears. In addition to providing for hardened steel gears, care must also be taken to provide enhanced support bearings for the various elements of the power tool gear train to ensure that they too are not damaged by the adverse loading conditions, and particularly, by shaft deflection.
In U.S. Pat. No. 3,973,449, it has been suggested that armature shaft deflection may be limited by supporting the distal end of the armature shaft in a bearing. However, this solution has been found to suffer from a number of disadvantages. The armature shaft is typically supported in a pair of bearings located, respectively, at each end of the armature winding. If a third bearing is added to support the distal end of the armature shaft, the armature shaft would then be supported in three bearings at spaced locations. Because of the tolerances inherent in the design of power tools, the accurate alignment of a single shaft in three spaced bearings is difficult. Misalignment will, of course, cause the development of friction and excessive heat in the bearings, leading to poor performance and potential failure of the device. Furthermore, the supporting of the distal shaft end in a bearing is difficult because the bearing is frequently located adjacent to an external housing wall typically made of plastic. The friction and heat generated in the bearing can lead to bearing failure and can lead to melting of the plastic housing in which the bearing is supported. Accordingly, it is, therefore, desirable to develop an improved durable mechanism for limiting shaft deflection with higher tolerances.